Update README changelog, remove Firebug IT branding, retire dead mobile middleware

README.md: backfill the Changelog with the 2026.9.5, 2026.8.31.1, 2026.8.31,
and v1.2.0 releases that were missing (it jumped straight from 1.0.0 to
1.1.0). Also correct the stale "Mobile App Support"/"Mobile App
Integration" sections - the native Android app is retired, and the
sync protocol they describe now powers the offline PWA instead.

Remove Firebug IT branding/contact info across README.md, API.md, and
the site footer, and drop the stale tasks.firebugit.com fallback from
development.py's ALLOWED_HOSTS/CSRF_TRUSTED_ORIGINS.

AllowMobileAppFramingMiddleware detected the native app via a
'com.firebugit.keepitgoing' User-Agent check to allow WebView iframe
embedding. With that app retired, replaced it with
SecurityHeadersMiddleware, which applies the same X-Frame-Options/CSP
headers unconditionally instead of only for non-mobile requests -
same protection for real users, dead branch and dead branding gone.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/base.html

@@ -142,7 +142,7 @@
         {% block auth_content %}{% endblock %}
     </div>
     <footer class="footer">
-        <p>&copy; 2025 Firebug IT. All rights reserved.</p>
+        <p>&copy; 2026 Keith Smith. All rights reserved.</p>
     </footer>
     {% endif %}
     {% endblock %}